U.S. patent number 5,146,609 [Application Number 07/486,049] was granted by the patent office on 1992-09-08 for cellular radio system having reduced handover disruption and method for use in such a system.
This patent grant is currently assigned to Motorola, Inc.. Invention is credited to Michael D. Kotzin, Daniel R. Tayloe.
United States Patent |
5,146,609 |
Tayloe , et al. |
September 8, 1992 |
Cellular radio system having reduced handover disruption and method
for use in such a system
Abstract
A handover technique in a cellular radio communication system
(100) for handing over a radio (102) from a first channel in a
first cell to a second channel in a second cell, the technique
comprising generating a fill-in message extrapolated from a message
of the radio on the first channel and communicating the fill-in
message when the radio has ceased communicating on the first
channel; and terminating the fill-in message when the radio begins
communicating on the second channel, thereby reducing handover
disruption. The messages may be digitized audio messages. A timer
(110, 118) may be employed to allow the fill-in message to
terminate at substantially the same time as the radio begins
communicating on the second channel. Alternatively, the fill-in
message may be directly terminated (FIG. 2) in response to the
radio beginning to communicate on the second channel. If the
messages are data messages, no fill-in message is generated and the
handover is completed directly (FIG. 2) in response to the radio
beginning to communicate on the second channel.
Inventors: |
Tayloe; Daniel R. (Arlington
Heights, IL), Kotzin; Michael D. (Buffalo Grove, IL) |
Assignee: |
Motorola, Inc. (Schaumburg,
IL)
|
Family
ID: |
23930409 |
Appl.
No.: |
07/486,049 |
Filed: |
February 27, 1990 |
Current U.S.
Class: |
455/436;
455/63.1 |
Current CPC
Class: |
H04W
36/38 (20130101); H04W 36/08 (20130101) |
Current International
Class: |
H04Q
7/38 (20060101); H04Q 7/30 (20060101); H04Q
007/00 (); H04B 001/00 () |
Field of
Search: |
;455/33,54,63,166,34
;379/59,60,63 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Kuntz; Curtis
Assistant Examiner: Belzer; Christine
Attorney, Agent or Firm: Hillman; Val Jean F.
Claims
We claim:
1. In a cellular radio communication system covering a plurality of
cells and having a plurality of base station radios each serving a
respective cell, a population of user radios which may communicate
on radio channels with the plurality of base station radios in the
plurality of cells, and a central controller linked to the
plurality of base station radios for communicating messages between
the central controller and the population of user radios, handover
means for causing one of the user radios to cease communicating on
a first channel in a first cell and to begin communicating on a
second channel in a second cell adjacent the first cell, the
handover means comprising:
fill-in means for generating a fill-in message extrapolated from a
message of the one of the user radios on the first channel and for
communicating the fill-in message when the one of the user radios
has ceased communicating on the first channel;
validating means, responsive to signals on the second channel, for
validating when the one of the user radios begins communicating on
the second channel in the second cell; and
terminating means, responsive to the validating means, for
terminating the fill-in message when the one of the user radios
begins communicating on the second channel in the second cell,
thereby reducing handover disruption.
2. A system according to claim 1 wherein the terminating means
terminates the fill-in message after a predetermined time from when
the one of the user radios ceases to communicate on the first
channel so as to terminate the fill-in message at substantially the
same time as the one of the user radios begins communicating on the
second channel.
3. A system according to claim 1 wherein the one of the user radios
includes means for validating its presence in the second cell with
the base station of the second cell after entering the second cell
and before communicating messages in the second cell, and the
terminating means terminates the fill-in message after a
predetermined time from when the one of the user radios begins
validating its presence in the second cell so as to terminate the
fill-in message at substantially the same time as the one of the
user radios begins communicating on the second channel.
4. A system according to claim 1 wherein the base station radio of
the second cell includes means for generating and transmitting to
the central controller a control signal when the one of the user
radios is not validly communicating a message on the second channel
in the second cell, and the terminating means terminates the
fill-in message in response to the cessation of the control signal
when the one of the user transceivers begins validly communicating
a message on the second channel in the second cell.
5. A system according to claim 1 wherein the messages are
communicated digitally.
6. A system according to claim 1 wherein the fill-in message is
generated digitally.
7. A system according to claim 1 wherein the handover means further
comprises means for establishing during handover a multi-party call
between a calling party and the first channel in the first cell and
the second channel in the second cell.
8. A system according to claim 1 wherein the fill-in message is an
audio message.
9. A system according to claim 8 wherein the fill-in means
comprises a transcoder.
10. A system according to claim 1 wherein the cellular radio
communication system is a cellular radio telephone system.
11. In a cellular radio communication system covering a plurality
of cells and having a plurality of base station radios each serving
a respective cell, a population of user radios which may
communicate on radio channels with the plurality of base station
radios in the plurality of cells, and a central controller linked
to the plurality of base station radios for communicating messages
between the central controller and the population of user radios,
handover means for causing one of the user radios to cease
communicating on a first channel in a first cell and to begin
communicating on a second channel in a second cell adjacent the
first cell, the handover means comprising:
means for generating and transmitting to the central controller a
fill-in message when the one of the user radios ceases
communicating on the first channel in the first cell;
validating means, responsive to signals on the second channel, for
validating when the one of the user radios begins communicating on
the second channel in the second cell; and
terminating means, responsive to the validating means, for
terminating the fill-in message and transmitting to the central
controller message information from the one of the user radios when
the one of the user radios begins validly communicating a message
on the second channel in the second cell, thereby reducing handover
disruption.
12. A system according to claim 11 wherein the message is a digital
data message.
13. A system according to claim 11 wherein the means for generating
and transmitting is a transcoder.
14. A system according to claim 11 wherein the cellular radio
communication system is a cellular radio telephone system.
15. A method of handover from a first channel in a first cell to a
second channel in a second cell in a cellular radio communication
system covering a plurality of cells and having a plurality of base
station radios each serving a respective cell, a population of user
radios which may communicate on radio channels with the plurality
of base station radios in the plurality of cells, and a central
controller linked to the plurality of base station radio
transceivers for communicating messages between the central
controller and the population of user radios, the method comprising
the steps of:
generating a fill-in message extrapolated from the message of the
one of the user radios on the first channel and communicating the
fill-in message when the one of the user radios ceases
communicating on the first channel in the first cell;
validating when the one of the user radios begins communicating on
the second channel in the second cell; and terminating the fill-in
message when the one of the user radios begins communicating on the
second channel, in the second cell, thereby reducing handover
disruption.
16. A method according to claim 15 wherein the terminating means
terminates the fill-in message after a predetermined time from when
the one of the user radios ceases to communicate on the first
channel so as to terminate the fill-in message at substantially the
same time as the one of the user radios begins communicating on the
second channel.
17. A method according to claim 15 wherein the one of the user
radios validates its presence in the second cell with the base
station radio of the second cell after entering the second cell and
before communicating messages in the second cell, and the fill-in
message is terminated after a predetermined time from when the one
of the user radios begins validating its presence in the second
cell so as to terminate the fill-in message at substantially the
same time as the one of the user radios begins communicating on the
second channel.
18. A method according to claim 15 wherein the fill-in message is
an audio message.
19. A method of handover from a first channel in a first cell to a
second channel in a second cell in a cellular radio communication
system covering a plurality of cells and having a plurality of base
station radios each serving a respective cell, a population of user
radios which may communicate on radio channels with the plurality
of base station radios in the plurality of cells, and a central
controller linked to the plurality of base station radio
transceivers for communicating messages between the central
controller and the population of user radios, the method comprising
the steps of:
generating and transmitting to the central controller a control
signal when the one of the user radios ceases communicating a
message on the first channel in the first cell;
generating a fill-in message extrapolated from the message of the
one of the user radios on the first channel and communicating the
fill-in message to the central controller when the one of the user
radios ceases communicating on the first channel in the first cell;
validating when the one of the user radios begins communicating on
the second channel in the second cell; and
terminating the control signal and the fill-in message and
transmitting to the central controller message information from the
one of the user radios when the one of the user radios begins
validly communicating a message on the second channel in the second
cell, thereby reducing handover disruption.
20. A method according to claim 19 wherein the fill-in message is a
digital data message.
Description
BACKGROUND OF THE INVENTION
This invention relates to cellular radio systems.
In a typical known cellular radio system a mobile radio unit in a
cell communicates via a radio channel with the cell's base station,
which in turn communicates with a fixed, land-based switch
connected to the landline telephone system. When the mobile radio
unit moves from one cell to another adjacent cell, a handover must
be performed, in which the mobile ceases communicating with the old
cell's base station on a first channel and begins communicating
with the new cell's base station on a second, different channel.
This handover process causes a disruption in communication to and
from the mobile unit. This disruption is normally experienced in
the form of an audio mute. Since the background noise is often
quite high in a mobile environment, a mute is usually quite
obvious. The coverage area of cellular radio systems heretofore has
been such that the cell size has been sufficiently large to produce
only occasional mutes which are not objectionable to most users.
However, as cells shrink in size (as they are expected to do with
digital cellular radio systems) handovers will occur much more
frequently and the consequent mutes will become proportionately
more disruptive to communication.
In existing digital cellular systems it is known to employ a
"fill-in" function in the event of missing speech blocks or as a
result of decoding errors or "stolen" speech blocks used for data.
In such a known system a "fill-in" audio signal is extrapolated
(using one of a variety of, known algorithms) from an immediately
preceding audio signal for a period of up to some 320 milliseconds
to fill in for missing audio.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an improved
cellular radio system and method for use in such a system wherein
handover disruption may be reduced.
In one preferred form there is provided audio handover means in a
cellular radio communication system covering a plurality of cells
and having a plurality of base station radios each serving a
respective cell, a population of user radios which may communicate
on radio channels with the plurality of base station radios in the
plurality of cells, and a central controller linked to the
plurality of base station radios for communicating messages between
the central controller and the population of user radios,
the handover means causing one of the user radios to cease
communicating on a first channel in a first cell and to begin
communicating on a second channel in a second cell adjacent the
first cell, the handover means comprising:
fill-in means for generating a fill-in message extrapolated from a
message of the one of the user radios on the first channel and for
communicating the fill-in message when the one of the user radios
has ceased communicating on the first channel;
validating means, responsive to signals on the second channel, for
detecting when the one of the user radios begins communicating on
the second channel; and terminating, responsive to the validating
means, means for terminating the fill-in message when the one of
the user radios begins communicating on the second channel, thereby
reducing handover disruption.
The fill-in messages may be digitized audio messages. The
terminating means may employ a timer to allow the fill-in message
to terminate at substantially the same time as the one of the user
radios begins communicating on the second channel. Alternatively,
the fill-in message may be directly terminated in response to the
one of the user radios beginning to communicate on the second
channel.
In another preferred form there is provided data handover means in
a cellular radio communication system covering a plurality of cells
and having a plurality of base station radios each serving a
respective cell, a population of user radios which may communicate
on radio channels with the plurality of base station radios in the
plurality of cells, and a central controller linked to the
plurality of base station radios for communicating messages between
the central controller and the population of user radios,
the handover means causing one of the user radios to cease
communicating on a first channel in a first cell and to begin
communicating on a second channel in a second cell adjacent the
first cell, the handover means comprising:
means for generating and transmitting to the central controller a
control signal when the one of the user radios is not validly
communicating a message on the second channel in the second cell;
and
terminating means for terminating the control signal and for
transmitting to the central controller message information from the
one of the user radios when the one of the user transceivers begins
validly communicating a message on the second channel in the second
cell.
BRIEF DESCRIPTION OF THE DRAWINGS
Two cellular radio systems, and methods for controlling handovers
therein so as to reduce handover disruption, in accordance with the
present invention will now be described, by way of example only,
with reference to the accompanying drawings, in which:
FIGS. 1(a), 1(b) and 1(c) are schematic block diagrams of part of a
first cellular radio system showing communication paths used prior
to, during, and after a handover; and
FIG. 2 is a schematic block diagram of part of a second cellular
radio system showing communication paths used prior to, during, and
after a handover.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to FIG. 1(a), a first digital cellular radio
telephone system 100 supports a population of cellular radio
telephones, such as mobile cellular radio telephone 102, which can
move in and between cells of the system. A first cell of the system
has a base terminal radio transceiver 104 which transmits radio
messages to and receives radio messages from cellular radio
telephones (such as mobile radio telephone 102) in its cell, on
predetermined radio channels under the control of a central
controller 106, including a timer 107. The base terminal radio
transceiver 104 communicates its cell's messages to and from the
central controller 106 via an associated transcoder 108 (which may
take the form of a conventional digital signal processor or DSP)
which performs the necessary encoding and decoding (such as
interleaving/de-interleaving, time-division
multiplexing/de-multiplexing, convolutional coding/decoding,
bit-rate conversion, etc.) to allow the first cell's messages to be
satisfactorily carried between the central controller 106 and the
base terminal radio transceiver 104. The transcoder 108 is provided
with a timer 110 and an extrapolator 112, whose function will be
described hereafter.
A second cell of the system has a base terminal radio transceiver
114 which transmits radio messages to and receives radio messages
from cellular radio telephones (such as mobile radio telephone 102)
in its cell, on predetermined radio channels under the control of
the central controller 106. Like the base terminal radio
transceiver 104, the base terminal radio transceiver 114
communicates its cell's messages to and from the central controller
106 via an associated transcoder 116, analogous to the transcoder
108 associated with the base terminal radio transceiver 104 in the
first cell. The transcoder 116 is provided with a timer 118 and an
extrapolator 120. The central controller 106 communicates messages
from radio telephones in the first and second cells and a switching
center (not shown) connecting to a land-line telephone system (also
not shown). Audio messages are transmitted in digitized form
between the mobile radio telephone 102 and the base terminal
transceivers 104 and 114 to provide better audio quality.
When the mobile radio telephone 102 is exclusively in the first
cell and is in use, audio messages are transmitted from the mobile
radio telephone 102 to the base terminal radio transceiver 104 and
from the base terminal radio transceiver 104 (via the transcoder
108) to the central controller 106 along path 122. At the same
time, audio messages are transmitted from the base terminal radio
transceiver 104 to the mobile radio telephone 102 and from the
central controller 106 (via the transcoder 108) to the base
terminal radio transceiver 104 along path 124. The audio messages
between the mobile radio telephone and the base terminal radio
transceiver 104 are carried in duplex on the same radio channel,
assigned under the control of the central controller 106. When the
mobile radio telephone 102 moves out of the first cell and into the
second cell, its audio messages are switched, under the control of
the central controller 106, so that audio messages are transmitted
from the mobile radio telephone 102 to the base terminal radio
transceiver 114 and from the base terminal radio transceiver 114
(via the transcoder 116) to the central controller 106 along path
126, and audio messages are transmitted from the base terminal
radio transceiver 114 to the mobile radio telephone 102 and from
the central controller 106 (via the transcoder 116) to the base
terminal radio transceiver 114 along path 128. The audio messages
between the mobile radio telephone 102 and the base terminal radio
transceiver 114 are carried in duplex on the same radio channel,
assigned under the control of the central controller 106. The radio
channel used for the audio messages of the mobile radio telephone
102 in the first cell is different from the radio channel used for
the audio messages of the mobile radio telephone 102 in the second
cell.
Handover of the mobile radio telephone 102 from the first cell to
the second cell is achieved in the following manner. When the
mobile radio telephone 102 is in the first cell and begins to move
out of the first cell towards the second cell, the central
controller 106 commands the mobile radio telephone 102 to cease
transmitting on the first channel. At the same time the central
controller 106 also sets up a three-party "conference" call for
audio messages to the mobile 102 by switching audio messages from
the central controller to the mobile radio telephone so that the
messages are transmitted both by the base terminal transceiver 104
on the first channel in the first cell and by the base terminal
transceiver 114 on the second channel in the second cell. The
central controller 106 at the same time also commands the
transcoder 108 associated with the base terminal transceiver 104 in
the first cell to start the timer 110, and to generate and transmit
to the central controller 106, in place of audio messages from the
mobile radio telephone 102, fill-in audio extrapolated from the
audio messages most recently received from the mobile radio
telephone 102. The fill-in audio messages are generated digitally
from the digitized audio most recently received from the radio
telephone 102 on the first channel. The generation of predicted
audio fill-in messages is well known and understood by those
skilled in the art and need not be described in further detail. The
mobile radio telephone immediately initiates a procedure of
"handshaking" with the base terminal transceiver 114 in the second
cell in order to validate its presence in the second cell, but no
audio path from the mobile radio telephone is yet established. This
condition is shown in FIG. 1(b).
The mobile radio telephone 102 continues to move out of the first
cell and enters the second cell. On completion of the "handshake"
validation procedure, the mobile radio telephone 102 begins
transmitting its audio messages in the second cell on the second
channel and these are communicated to the central controller 106
along the path 126. "Handshake" validation procedures are well
known and understood by those skilled in the art and need not be
described in further detail. The transcoder 108 continues to
generate and transmit fill-in audio until the timer 110 reaches a
predetermined count, at which time the fill-in audio message is
terminated. The central controller 106 then terminates the
three-party "conference" call for audio messages from the central
controller 106 to the mobile radio telephone 102, leaving audio
messages between the mobile radio telephone 102 and the central
controller 106 supported only on the second channel in the second
cell along the paths 126 and 128. This condition is shown in FIG.
1(c).
The predetermined value to which the timer 110 is allowed to count
before the fill-in audio is terminated is chosen so that the
fill-in audio is terminated only after the mobile radio telephone
102 begins transmitting its audio messages on the second channel in
the second cell, so as to avoid a gap between the end of fill-in
audio and audio transmitted from the mobile radio telephone 102 on
the second channel in the second cell.
It will be appreciated that in this way the party being called from
the mobile radio telephone 102 perceives little or no disruption in
audio messages from the mobile radio telephone 102, since the
called party will receive fill-in predicted audio during the time
the handover is occurring and will receive new audio when the
handover has been completed.
It will be appreciated that if the audio path switch is timed (e.g.
controlled by the timer 107 in the central controller) and can be
made to occur at substantially the same time as new audio is
transmitted on the second channel in the second cell, the
three-party "conference" call used in the above example may be
eliminated.
In an alternative method to that described above of operating the
first cellular radio system, handover of the mobile radio telephone
102 from the first cell to the second cell is achieved in the
following manner. When the mobile radio telephone 102 is in the
first cell and begins to move out of the first cell towards the
second cell, the central controller 106 commands the mobile radio
telephone 102 to cease transmitting on the first channel. At the
same time the central controller 106 also commands the transcoder
108 associated with the base terminal transceiver 104 in the first
cell to generate and transmit to the central controller 106, in
place of audio messages from the mobile radio telephone 102,
fill-in messages extrapolated from the audio messages most recently
received from the mobile radio telephone 102. In this alternative
method of operation, the central controller 106 does not set up a
three-party "conference" connection, nor does it initiate the timer
110 in the transcoder 108.
The mobile radio telephone 102 continues to move out of the first
cell and enters the second cell. The mobile radio telephone
initiates a procedure of "handshaking" with the base terminal
transceiver 114 in the second cell in order to validate its
presence in the second cell. The initiation of this "handshake"
validation procedure is communicated to the central controller 106,
which then initiates the timer 107 in the central controller 106.
On completion of the "handshake" validation procedure, the mobile
radio telephone 102 begins transmitting its audio messages in the
second cell on the second channel and these are communicated to the
central controller 106 along the path 126. Meanwhile, the
transcoder 108 generates and transmits fill-in audio. When the
timer 107 reaches a predetermined count, the central controller 106
then completes the handover switching at that time, and terminates
the fill-in audio generated by the transcoder 108, leaving audio
messages between the mobile radio telephone 102 and the central
controller 106 supported only on the second channel in the second
cell along the paths 126 and 128.
The predetermined value to which the timer 107 is allowed to count,
in this alternative method of operation, before the fill-in audio
is terminated is less than that for the transcoder timer 110 in the
method of operation described above since the central controller
timer 107 is now initiated later than was the transcoder timer 110.
This later initiation of the central controller timer 107 allows
the much more reliable prediction of the remaining interval of time
before the mobile radio telephone 102 begins transmitting its audio
messages on the second channel in the second cell, since this is
now simply the time between initiation of the "handshake"
validation procedure and the initiation of true audio transmission.
Thus, it will be appreciated that this alternative method of
handover affords more reliable reduction of handover
disruption.
Referring now to FIG. 2, a second digital cellular radio telephone
system 200 supports a population of cellular radio telephones, such
as mobile cellular radio telephone 202, which can move in and
between cells of the system. A first cell of the system has a base
terminal radio transceiver 204 which transmits radio messages to
and receives radio messages from cellular radio telephones (such as
mobile radio telephone 202) in its cell, on predetermined radio
channels under the control of a central controller 206. The base
terminal radio transceiver 204 communicates its cell's messages to
and from the central controller 206 via an associated transcoder
208 (which may take the form of a conventional digital signal
processor or DSP) which performs the necessary endcoding and
decoding to allow the first cell's messages to be satisfactorily
carried between the central controller 206 and the base terminal
radio transceiver 204. The transcoder 208 is provided with a
control signal generator 210 and an extrapolator 212, whose
function will be described hereafter
A second cell of the system has a base terminal radio transceiver
214 which transmits radio messages to and receives radio messages
from cellular radio telephones (such as mobile radio telephone 202)
in its cell, on predetermined radio channels under the control of
the central controller 206. Like the base terminal radio
transceiver 204, the base terminal radio transceiver 214
communicates its cell's messages to and from the central controller
206 via an associated transcoder 216, analogous to the transcoder
208 associated with the base terminal radio transceiver 204 in the
first cell. The transcoder 216 is provided with a control signal
generator 218 and an extrapolator 220. The central controller 206
communicates messages from radio telephones in the first and second
cells and a switching center (not shown) connecting to a land-line
telephone system (also not shown). Audio messages are transmitted
in digitized form between the mobile radio telephone 202 and the
base terminal transceivers 204 and 214 to provide better audio
quality.
When the mobile radio telephone 202 is exclusively in the first
cell and is in use, audio messages are transmitted from the mobile
radio telephone 202 to the base terminal radio transceiver 204 and
from the base terminal radio transceiver 204 (via the transcoder
208) to the central controller 206. At the same time, audio
messages are transmitted from the base terminal radio transceiver
204 to the mobile radio telephone 202 and from the central
controller 206 (via the transcoder 208) to the base terminal radio
transceiver 204. The audio messages between the mobile radio
telephone and the base terminal radio transceiver 204 are carried
in duplex on the same radio channel, assigned under the control of
the central controller 206. When the mobile radio telephone 202
moves out of the first cell and into the second cell, its audio
messages are switched, under the control of the central controller
206, so that audio messages are transmitted from the mobile radio
telephone 202 to the base terminal radio transceiver 214 and from
the base terminal radio transceiver 214 (via the transcoder 216) to
the central controller 206, and audio messages are transmitted from
the base terminal radio transceiver 214 to the mobile radio
telephone 202 and from the central controller 206 (via the
transcoder 216) to the base terminal radio transceiver 214. The
audio messages between the mobile radio telephone 202 and the base
terminal radio transceiver 204 are carried in duplex on the same
radio channel, assigned under the control of the central controller
206. The radio channel used for the audio messages of the mobile
radio telephone 202 in the first cell is different from the radio
channel used for the audio messages of the mobile radio telephone
202 in the second cell.
Handover of the mobile radio telephone 202 from the first cell to
the second cell is achieved in the following manner. When the
mobile radio telephone 202 is in the first cell and begins to move
out of the first cell towards the second cell, the central
controller 206 commands the mobile radio telephone 202 to cease
transmitting on the first channel. At the same time the central
controller 206 also switches audio messages to the mobile 202 so
that the messages are transmitted both by the base terminal
transceiver 204 on the first channel in the first cell and by the
base terminal transceiver 214 on the second channel in the second
cell. The central controller 206 at the same time also commands the
transcoder 208 associated with the base terminal transceiver 204 in
the first cell to generate and transmit to the central controller
206, in place of audio messages from the mobile radio telephone
202, fill-in audio extrapolated from the audio messages most
recently received from the mobile radio telephone 202. Also at the
same time the central controller 206 commands the code signal
generator 218 in the transcoder 216 associated with the base
terminal radio transceiver 214 to generate and transmit to the
central controller 206 and predetermined code signal (e.g. a
constant, known PCM value).
The mobile radio telephone 202 continues to move out of the first
cell and enters the second cell. The mobile radio telephone
initiates a procedure of "handshaking" with the base terminal
transceiver 214 in the second cell in order to validate its
presence in the second cell and, on completion of the "handshake"
validation procedure, the mobile radio telephone 202 begins
transmitting its audio messages in the second cell on the second
channel. When the base terminal transceiver 214 receives audio
messages on the second channel, it terminates the generation and
transmission of the control signal by control signal generator and,
in its place, communicates the received audio to the central
controller 206. As soon as the central controller 206 begins to
receive from the base terminal transceiver 214 audio instead of the
control signal, it commands the extrapolator 212 in the transcoder
208 associated with the base terminal radio transceiver 204 to
terminate the fill-in audio and completes the handover switch,
leaving audio messages between the mobile radio telephone 202 and
the central controller 206 supported only on the second channel in
the second cell.
It will be appreciated that in the system of FIG. 2 high
synchronicity of switching between fill-in and true audio is
reliably achieved without dependence on timers and predetermined
timing periods since the switching is directly responsive to the
reception by the base terminal transceiver 214 of audio on the
second channel in the second cell.
It will be appreciated that in this way the party being called from
the mobile radio telephone 202 perceives little or no disruption in
audio messages from the mobile radio telephone 202, since the
called party will receive fill-in predicted audio during the time
the handover is occurring and will receive new audio without
significant delay when it is available.
As explained above, when the messages transmitted by the mobile
radio telephone 202 are audio messages, during handover when "real"
audio is missing the audio can be predicted by extrapolation to
produce a fill-in audio signal which can be used in place of
missing audio to minimize handover disruption. However, if the
messages transmitted by the mobile radio telephone 202 are data
messages, it is not possible satisfactorily to extrapolate or
predict for missing audio during handover. In this case, the system
of FIG. 2 is made to operate in an analogous manner to that
described above, but during a handover from the first cell to the
second cell the extrapolator 212 is not made to generate and
transmit a fill-in signal. In all other respects the handover is
conducted in the same manner as that described above, with the code
signal generator 218 in the transcoder 216 generating and
transmitting to the central controller 206 a control signal until
the mobile radio telephone 202 begins transmitting messages on the
second channel in the second cell. In this way handover disruption
of data messages is minimized, since the transmission gap (during
which the mobile radio telephone 202 does not transmit data while
being handed over from the first cell to the second cell) is kept
to a minimum.
It will be appreciated that although in the above examples the
extrapolators 112, 120, 212 and 220 and the timers 110 and 118 have
been described as being located within their associated respective
transcoders, these components could alternatively be located at the
respective central controller or at any other convenient
location.
It will also be understood that the central controller 106 or 206
could be implemented in a conventional base site controller or
mobile switching center, or its functions could be divided between
a base site controller and a mobile switching center.
It will be appreciated that various other modifications or
alternatives to the above described embodiment will be apparent to
the man skilled in the art without departing from the inventive
concept.
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